1. Field of the Invention
The present invention relates to a porous pipe preferably used as, for example, a drain pipe for a culvert, and relates to an apparatus for producing the pipe and a method of producing the pipe.
2. Description of the Related Art
A method of forming a number of holes in a pipe wall made of synthetic resin, conventionally, includes perforating a nonporous pipe by moving a drill in a forward and rearward direction or previously producing a perforated tape and forming a pipe by spirally winding the tape during a separate step.
FIG. 13 shows an example of a conventional porous pipe formed by spirally winding a tape. In the drawing, reference number 31 designates a flat tape made of resin and previously aligned with a number of holes 32 in a longitudinal direction. Reference number 33 designates a corrugated tape having a section in a shape of a channel-like groove and integrally provided with connection pieces 34 directed outwardly at both front ends in a cross direction thereof. Further, by overlapping the flat tape 31 and the corrugated tape 33 such that the two end edges of the flat tape 31 overlap the connection pieces 34 of the corrugated tape 33, and by winding both members spirally while melting together or adhering together both members, there is formed a corrugated pipe having the holes 32 at valley portions of an outer periphery thereof.
FIG. 14 shows an enlarged portion of a hole of the porous pipe formed by the conventional method, in which the hole 32 is formed substantially orthogonally to a pipe wall 35, and in which an inner peripheral face of the hole 32 and a surface of the pipe wall 35 are substantially orthogonal to each other. An arrow mark of the drawing designates a flow of water into the hole 32.
In the above-described conventional methods, according to the former method of perforating the nonporous pipe hole-by-hole using the drill, there is a drawback in that there is a limit to the speed of moving the drill and to the perforating speed, that is, production speed of a porous pipe cannot be increased. Further, this method poses a problem of creating a poor operational environment such as producing drilling chips or emitting noise. In addition, this method also poses a problem in that the pipe wall is warped during drilling and, therefore, the strength of the pipe is reduced.
Meanwhile, according to the latter method of forming holes when producing the tape, a special apparatus capable of forming holes is required when forming such a tape. Further, production of the tape and forming of the pipe are carried out by separate steps. Therefore, there is a problem in that the number of steps is increased and production cost is increased. Further, a place for storing the tape before being employed in a pipe forming machine is needed, and control therefore is also needed.
In addition, according to the corrugated porous pipe formed by being wound spirally, as shown by FIG. 13, the end edge of the perforated tape 31 overlaps the tape 33 on the other side, and the tape portions are melted together or adhered together. Therefore, there is a drawback in that a portion of the perforated tape 31 is restricted by an amount of the overlapped portion, the porosity cannot be increased, and an absorbing or draining function is deteriorated by that amount.
Further, as shown in FIG. 14, according to the conventional porous pipe, the surface of the pipe wall 35 surrounding the hole 32 is made substantially orthogonal to a peripheral wall of the hole 32. Therefore, when the porous pipe is used for a drain pipe of a culvert or the like, there is a case in which water does not necessarily flow smoothly to the hole 32, and the absorbing function is not sufficient.